This invention relates generally to processing signals output from an interferometer sensor array. This invention relates particularly processing signals output from a fiber optic interferometer sensor array arranged to monitor a physical parameter by means of phase changes in such an array when it is exposed to the parameter. Still more particularly, this invention relates to processing signals output from an array of tri-cell photodetector array used to detect signals output from an interferometer sensor array.
Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in a parameter such as fluid pressure, acceleration, magnetic field intensity, etc. Such sensing elements measure the time-varying phase delay between optical signals that have propagated along separate optical paths having unequal path length.
Mixing between a reference signal and a data signal is often necessary to extract information from an optical carrier. In interferometric sensing the mixing is typically between a reference signal and a signal whose phase has been modified, or modulated by the parameter being measured.
Modulation is commonly used to transmit information from an information source, such as a sensor system where information is detected, to an information destination, such as a receiver system where detected signals arc received and processed. According to conventional modulation techniques, a signal of interest detected by a sensor modulates a carrier signal by modifying one or more characteristics of the carrier signal, such as amplitude, frequency or phase, to form a modulated carrier signal. The modulated carrier signal is then more easily transmitted over the appropriate communication channels to the destination or receiver system where the modulated carrier signal is demodulated to recover the signal of interest and determine the information.
One type of sensor system that employs modulation techniques includes fiber optic sensors, for example, as used in fiber optic interferometers for distance measurements. The fiber optic sensors detect or sense light signals that modulate the output phase of the sensor system or interferometer. The modulated carrier can then be photodetected and transmitted to a receiver system. In a system having an array of sensors, the signals are often multiplexed, for example, using time division multiplexing (TDM) and/or wavelength division multiplexing (WDM).
Fiber optic sensor systems acquire in the demodulation process a signal component proportional to the sine of the sensor phase shift and another signal component proportional to the cosine of the phase shift. The sine of the sensor phase shift is referred to as the quadrature term, Q; and the cosine of the sensor phase shift is referred to as the in-phase term, I. The angle of the phase shift is determined by calculating ratio Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arctangent routine to find the sensor phase shift.
One type of modulation technique used in interferometers and other sensor systems uses phase generated carriers. The sensor's time varying phase signal (signal of interest) modulates the phase generated carriers to form a modulated carrier. Both the signal of interest and the phase generated carriers can be mathematically represented as a Bessel series of harmonically related terms. During modulation, the Bessel series of the signal of interest modulates the Bessel series of the phase generated carrier. The number of terms in the Bessel series of the resulting modulated carrier will be dependent upon the level of the measured or detected signal of interest. The harmonically related terms in the Bessel series of the modulated carrier represent both the measured or detected signal of interest and the carrier signal.
Typical fiber optic sensor systems using phase generated carriers to transmit a detected or measured signal (signal of interest) to a receiver system have used a pair of quadrature carriers with frequencies of either ωc and 2ωc or 2ωc and 3ωc, where ωc is the phase generated carrier frequency. In multiplexed sensor systems, the sensor sampling frequency fs must be selected to ensure that frequencies greater than fs/2 are not aliased into the band of interest below fs/2.
In some systems the optical signal input to the interferometer is a phase generated carrier produced by producing time-dependent variations in the frequency of the optical signal output by a laser. A phase generated carrier may be produced by various techniques. One technique involves routing the source output through a phase modulator and applying a sequence of separate and different linear ramp voltages to the linear phase modulator to produce step changes in the optical frequency.
Another technique for producing a phase generated carrier uses sinusoidal phase modulation of the source signal. Instead of sampling signals associated with separate optical frequencies, the sampling of signals is associated with integration over portions of a period of the phase generated carrier.
Still another technique for producing a phase generated carrier involves the use of a numerically controlled oscillator (NCO). A problem with using an NCO in an array comprising a plurality of interferometric sensors is an uncertainty in the sign of the sensor response under certain conditions. In particular, carriers that are 180° out of phase with the NCO phase will produce sensor responses with opposite sign after demodulation different than those produced by carriers that are in phase with the NCO phase. When coherently combined, sensor responses with opposite signs will combine destructively, which results in an attenuation of the combined output and a reduction in overall system dynamic range.
A significant problem in systems that employ the reception of optical signals from an optical fiber is signal fading caused by changes in the polarization of the optical signals transmitted through the optical fiber. Specifically, phase information from two or more optical signals propagated through a fiber optic transmission line can be lost at the receiver if the polarizations of two signals of interest are crossed, resulting in no detector beat note. It is therefore necessary to provide some mechanism for treating the signal that yields a suitably large detector beat note for signal processing in all cases of polarization wander.
Polarization diversity detectors are used to detect an optical signal of uncertain polarization and produce an electrical output corresponding to a selected polarization component in the optical signal. U.S. Pat. No. 5,852,507, which issued Dec. 22, 1998 to David B. Hall and which is assigned Litton Systems, Inc., assignee of the present invention, discloses a tri-cell polarization diversity detector that produces multiple output signals from an incident beam that has two orthogonal polarization components. The disclosure of U.S. Pat. No. 5,852,507 is incorporated by reference into the present disclosure.
U.S. Pat. No. 5,448,058, which issued Sep. 5, 1995 to Arab-Sadeghabadi and vonBierein and which is assigned Litton Systems, Inc., assignee of the present invention, discloses a polarization diversity detector that includes an array of three polarizers having axes of polarization spaced apart by selected angles such that an optical signal incident on the polarizer array has a component along at least one of the axes of polarization. A photodetector array is arranged such that each photodetector receives light from a selected one of the polarizers. At least one of the photodetectors receives parallel polarization components that form an electrical signal that indicates interference between the parallel polarization components. The disclosure of U.S. Pat. No. 5,448,058 is incorporated by reference into the present disclosure.